Even without seeing the files, I'd lay long odds that the STL files aren't manifold, which means they're not closed surfaces. Different slicers (try to) perform automatic repairs in different ways, but the results generally differ from the original objects, sometimes in hilarious ways.

As a rule of thumb, you should (ideally) figure out how to create models that produce valid STL files or (if possible) manually repair the broken files before attempting to slice them.

101? I guess I was allowed to skip that class. Seven years printing with four different printers, and this was the first time I had seen this. I have used SolidWorks since 2001; not much experience with crappy, online CAD software.

Phil wrote:101? I guess I was allowed to skip that class. Seven years printing with four different printers, and this was the first time I had seen this. I have used SolidWorks since 2001; not much experience with crappy, online CAD software.

You can skip it when you're making your own files, using competent software

Custom 3D printing for you or your business -- quote [at] pingring.org

I have seen quite a few files from high end software but saved as different file types that end up non-manifold. Getting the native file(if changes are needed) and the stl usually works best. Usually an easy fix if there are problems but previewing the gcode is a must.